1 .' Under' is used to talk about something which is covered by something else. It can be used when discussing about layers of something, for numbers, age and also for showing a specific expression.
Ex - I completed this activity under 7 hours. / Dolphin resides under the sea.

2. 'Below' is when the meaning is “less than / at a lower position or level.”
Ex - It is 18 degrees below zero.

3. 'Beneath' is used in formal language, it’s meaning is same as under. It also indicates something covered by something.
Ex - In an emergency water landing, you will find a floating device beneath your seat cushion.

4. 'Underneath' is a casual version of under. It can be used instead of under while giving a little more emphasis for describing a place.
Ex - I have kept that paper underneath the sofa.